Commercial water systems generally rely on the use of steel components for transporting water. Accordingly, a major concern in the operation of such systems is the control of steel corrosion. Steel surfaces exposed to water eventually form a protective oxide film that reduces the rate of metal corrosion. However, since oxide formation is relatively slow, severe pitting of a fresh surface can occur. Further, bare metal surfaces are continually forming on mechanical components such as pump pistons and pump impellers that move rapidly or suddenly. At these surfaces small gas bubbles develop and then violently collapse causing destruction of the oxide film. During the time when the oxide layer reforms, cavitation corrosion (pitting) can occur.
The value of nitrite (NO.sub.2) as a cooling system corrosion inhibitor for steel is well recognized because nitrite increases the rate of iron oxide formation on fresh steel surfaces. This is particularly important for protecting mechanical parts whose oxide layers are constantly being stripped away. It is therefore desirable to always maintain proper nitrite levels in a cooling system being treated with a nitrite inhibitor.
There are three common problems associated with maintaining proper treatment levels in a closed cooling system. First, improper maintenance of system treatment levels is often encountered due to neglect. Closed systems are often assumed to be self-sufficient, resulting in infrequent inhibitor level checks by operators. Also, system leaks reduce nitrite treatment levels forcing operators to quickly apply corrections. Secondly, where bacterial infestation is encountered, nitrite is consumed as a food source, thereby reducing active nitrite level. A low nitrite level can falsely indicate low product feed rate. Increasing feed rate without biocide control would only further nourish the bacteria causing them to further multiply. Thus, the pH of the cooling water must also be monitored at times to obtain information on bacterial content. Thirdly, uncontrolled water loss such as leaks reduce nitrite treatment levels forcing the operator to quickly apply corrections. This may result in inconsistent production or system operation.
Since nitrite is continually being consumed both as a corrosion inhibitor and as a bacterial food source, inert tracers cannot be used reliably to control product level. Nitrite itself must be monitored.
An example of a closed cooling system is water cooling rollers in a continuous steel casting machine where water is circulated under pressure between a heat exchanger and the rollers. To control corrosion in such a closed system, a nitrite-containing compound is employed as a corrosion inhibitor, although other corrosion inhibitors may be used. Knowing the significant parameters in a system, such as water quality, water usage rate, and system metallurgy, the standard concentration for nitrite to inhibit corrosion can be determined.
Heretofore, it has been known to analyze nitrite concentration within a closed water cooling system with a titrimetric analyzer. More specifically, a field programmable analyzer (Model 301) made by Tytronics, Inc. has been programmed and slightly modified to analyze for nitrite in a closed water cooling system. The titrator was modified to produce an output signal referenced against a set point to control the operation of a feed pump and the feeding of nitrite to the cooling system. Operation of the titrator requires administration of a reagent for oxidizing nitrite. An oxidation-reduction-potential (ORP) electrode measures the potential in the testing cell. At a certain value, corresponding to the titration end point, the volume of titrant added is proportional to the analyte concentration. If the measurement is below a set point, the feed pump is energized to feed nitrite to the cooling system. This mode continues until a further measurement is taken which detects the concentration level to be above the set point, thereby causing the feed pump to be de-energized. Thereafter, when a further measurement is taken indicating the concentration level to be below the set point, the feed pump is again energized to feed nitrite into the cooling system.
This titrimetric technique of measuring nitrite concentration requires a cell construction providing a well defined sample volume. The analysis times are rather long and, of course, a reagent is necessary in order to perform a titration. Further, the commercial titrator is very costly and requires frequent maintenance.